The hydrocracking process is used to upgrade heavy oil fractions or feedstocks, such as heavy atmospheric gas oil, atmospheric resid, and vacuum gas oil, obtained from crude oil to more valuable lower molecular weight or lower boiling products, such as diesel, kerosene and naphtha. The heavy oil fraction that is typically hydrocracked comprises hydrocarbon components boiling above 290° C. (550° F.) with at least 90 weight percent of the heavy oil fraction boiling above 380° C. (716° F.). The heavy oil fraction may also contain asphaltene and polycyclic aromatic hydrocarbon components. A typical heavy feedstock has an initial boiling point above about 315° C. (600° F.) and a final boiling point below about 590° C. (1094° F.).
The polycyclic aromatic (“PCA”) hydrocarbons referred to herein are also known as poly-aromatic hydrocarbons or polycyclic aromatic hydrocarbons. A polycyclic aromatic hydrocarbon is a molecule that comprises three or more fused aromatic rings. The aromatic ring moieties of the PCA molecule can include rings having from four to seven carbon members. The most common ring size are those having five or six carbon members and many of the PCA molecules are composed only of six-member rings. Normally, the PCA molecules do not contain heteroatoms or carry substituents. The PCA molecules have a molecular weight falling within the range of from 400 to 1500 and boiling temperatures within the boiling range of the heavy feedstock.
The asphaltenes referred to herein include molecular components of the heavy feedstock that primarily consist of carbon, hydrogen, nitrogen, oxygen and sulfur atoms, and that are insoluble in n-heptane (C7H16) and soluble in toluene (C6H5CH3). Thus, the asphaltene component of the heavy feedstock is the hydrocarbon fraction that precipitates when n-heptane is added to it.
Hydrocracking is accomplished by contacting in a hydrocracking reaction vessel or zone the heavy feedstock with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield the upgraded products. The product upgrading is accomplished by cracking the larger hydrocarbon molecules of the heavy feedstock and adding hydrogen to the cracked molecules to yield lower molecular weight molecules.
The per-pass conversion across the hydrocracker reactor of the heavy feedstock depends on a variety of factors, including, for example, the composition of the heavy feedstock, the type of hydrocracking catalyst used, and the hydrocracker reactor conditions, including, reaction temperature, reaction pressure and reactor space velocity.
The hydrocracker reactor product is passed to a separation system that typically includes a fractionator or stripper that provides for separating the hydrocracker reactor product to yield at least one lower boiling conversion product and a fraction which comprises the portion of the heavy feedstock that is not converted to lower boiling products. The fraction of heavy feedstock that is not converted can include the asphaltenes and PCAs contained in the heavy feedstock and heavy PCAs that are formed as side products during the hydrocraking of the heavy feedstock. The separated fraction of unconverted heavy feedstock may be returned as a heavy oil recycle feed to the hydrocracker reactor.
One problem that is sometimes encountered in the processing of certain types of heavy and aromatic hydrocracker feedstocks is that the higher severity hydrocracker reactor conditions needed to provide for a desired high conversion can result in formation of heavy polycyclic aromatic side products that accumulate in the heavy oil recycle stream of the process. Additionally, in order to achieve the desired conversion of certain heavy hydrocracker feedstocks, the rate of heavy oil recycle often needs to be higher than that typically required when processing other types of feedstock. The combination of the formation of heavy polycyclic aromatics and higher recycle rates can cause an undesirable buildup of heavy polycyclic aromatics in the heavy oil recycle stream. This buildup can cause numerous problems in the operation of a hydrocracking process, such as, for example, increasing the rate of catalyst deactivation, reducing conversion yields, and causing equipment fouling.
Accordingly, there is a need for an improved hydrocracking process that provides for the high conversion hydrocracking of heavy hydrocarbon feedstocks and the reduction of buildup of heavy poly-aromatic hydrocarbons in the heavy oil recycle stream of the hydrocracking process.